Adhesives find use in an unending variety of applications, and are an integral part of our daily lives. A good adhesive must have many properties, some of which are mutually exclusive, and therefore require compromise in selection. For example, it must adhere tenaciously to the substrate upon which it is to be bonded, it must be able to be converted from a `non-adhesive` state to an adhesive state, it must be able to be properly applied or located on the substrate of choice, and must meet numerous other criteria such as cost, environmental resistance, toxicity, reliability, shear strength, peel strength, color, viscosity, etc. Of these, we are concerned here with the ability to be properly applied or located on the substrate of choice and the ability to be converted from a `non-adhesive` state to an adhesive state.
Conventional adhesives are applied in myriad ways, for example, dispensing, pouring, roller coating, spraying, dipping, painting, stenciling, and electrostatic coating. Each of these are performed manually or by machine. Those skilled in the art will appreciate that each technique poses problems in uniform and accurate application, and can be messy. Creation of complex patterns of adhesive is difficult, and is typically achieved by stenciling or dispensing. Both of these methods have a limit to their usefulness when trying to achieve fine resolution and accurate placement of the adhesive.
Conventional adhesives, such as ultraviolet (UV) light-curable adhesives, cannot be patterned with existing photodelineation methods. To do so would completely cure the adhesive, thereby rendering it useless as an adhesive. The UV adhesives must also be fully exposed to be cured, which requires that either the part to be bonded is very small, or it is transparent to UV light, so that the adhesive can be cured under the part. This severely limits their use.
Photoresists have been used as masks to create very fine patterns on printed circuit boards and semiconductor wafers. These polymers are typically applied as a dry film or a liquid, and are selectively exposed, and developed. While these materials are widely used in the electronics industry, they suffer from poor adhesion to the substrate and cannot be used as true adhesives to bond articles together due to the need to irradiate the adhesive to cure it. For example, Gurtler, in U.S. Pat. No. 3,909,930 describes the use of RISTON photoresist to create a cavity in a liquid crystal display. Those skilled in the art will appreciate that while RISTON is an excellent photoresist for masking and etching circuit patterns, it can not be used as an adhesive due to the marginal adhesion to the substrate. Sullivan, in U.S. Pat. No. 4,966,827 describes a cumbersome method to overcome the stated poor adhesion of dry film resists. Further, the resist must be fully and uniformly exposed by the light for proper results, thereby precluding its use as an adhesive to bond two opaque materials together. For example, RISTON cannot be used to adhesively bond a metal heat sink to a printed circuit board, because it is a very poor adhesive, and because the light cannot get through the opaque metal to initiate the curing photoreaction. Attempts to thermally cure these types of resists produce minimal, if any, adhesive bonding.
Therefore, a need has existed for a material and a method to produce fine line, high resolution patterns of adhesives to bond opaque materials together.